Devices for converting the frequency of electrical power are known in the art. One class of devices to perform this function are inverters, which convert DC energy to AC electrical energy. To enable the device to convert power from one frequency to an adjustable or controllable frequency, a rectifier is added to the inverter so as to first convert the applied power to direct current, and then employing the inverter, produce power of controllable frequency. The present invention relates to improvements in inverters.
FIG. 1 illustrates a typical prior art inverter which is supplied by direct current, and which, for producing three-phase power of controllable frequency, employs six thyristors. An inverter of the sort shown in FIG. 1 produces a six-step approximation to a three-phase sinusoidal voltage. The thyristors associated with each phase of the phases A,B and C, alternately connect the phase to either the positive or negative supply potential as determined by the firing logic (not illustrated). Also omitted from FIG. 1 are the commutation provisions for momentarily reversing current flow through the thyristor which is necessary to quench its conduction for turn-off. Turn-off, or commutation, is an essential provision in inverters of the type shown in FIG. 1, for, if turn-off fails to occur, for example, at T.sub.a, prior to turn-on of the complementary element T'.sub.a' a direct short circuit across the supply results. This phenomenon is referred to as "short through." Practical examples of an inverter of the type shown in FIG. 1 are found in "Solid-State Adjustable Frequency AC Drives" by P. G. Mesniaeff, appearing in Control Engineering, November 1971, page 57 et seq; see in particular page 66 and U.S. Pat. Nos. 3,639,819 and 3,781,641.
FIG. 2 illustrates a prior art arrangement for precluding short-through. This is accomplished by dividing the load windings (FIG. 2 shows only a single divided load winding) into two windings A and A', and connecting them for alternate energizaiton by their respective thyristors T.sub.a and T'.sub.a. Each winding is energized on only a half-wave basis, and in the event a thyristor fails to quench or turn-off, the impedance of the load winding remains effective for limiting the current and its rate of rise to moderate levels as compared to the short-through produced by FIG. 1, in the case of failure of a thyristor to turn-off. Furthermore, FIG. 2 illustrates the commutation provision, by illustrating the capacitor C, and the two steering or isolation diodes D.sub.a and D'.sub.a.
The operation of FIG. 2 is simply explained; when T.sub.a is conducting, the left-hand terminal of the capacitor is effectively joined to the negative supply potential but because the load winding is inductively coupled to the load, and oppositely poled, the diode D'.sub.a will be at twice the positive supply potential. The diode conducts and this double charge is trapped in the capacitor C. To commutate the current form load winding A to load winding A', it is necessary only to enable the thyrisistor T'.sub.a by pulsing its gate electrode. The trapped capacitor charge momentarily supplies the load current traversing the winding A, also draining off the internal junction charge within T.sub.a allowing it to turn-off and conduction to shift to T'.sub.a' and load winding A'. This results in the diode D.sub.a being elevated to twice the supply potential, and the capacitor charges in the opposite direction, ready for the next commutation from T'.sub.a to T.sub.a. The conduction pattern, and the sine wave approximation is shown in FIG. 3. Practical examples of inverters including divided load windings can be found in Hubner U.S. Pat. No. 3,887,859 and Greenwell U.S. Pat. No. 3,753,062; Graham, in U.S. Pat. No. 3,624,472, shows a similar arrangement in a cyclo-converter.
The prior art also includes arrangements in which the winding of a polyphase load, for example, a motor, are interconnected so that two load windings and their associated thyristors are always in series. Such an arrangement is shown in FIG. 4, for example, for a two-phase load with divided or complementary windings. As shown in FIG. 4, the windings of one phase are divided and are illustrated as windings A and A', associated with each of these windings a respective thyristors T.sub.a and T'.sub.a. Likewise, another phase comprises the divided windings B and B' with their associated thyristors T.sub.b and T'.sub.b. As illustrated in FIG. 4, any circuit path across the supply potential includes at least two windings and two thyristors. At times, an interphase reactor is inserted at a point common to all the windings, i.e., at point P. The two halves of the circuit of FIG. 4 operate essentially independently as described in connection with FIG. 2, but the consequences of a commutation failure are further diminished by virture of the serial feed and certain construction convenience advantages occur. In order to generate a two-phase output, the gating pulses for T.sub.b and T'.sub.b occur 90 electrical degrees after the corresponding gating pulses for T.sub.a and T'.sub.a, as illustrated in FIG. 5. In this connection, it should be noted that the thyristors for any phase are energized periodically, and alternately in sequence. The 90.degree. reference merely indicates that the gating pulses for one phase are delayed, with respect to the gating pulses for the other phase, by a length of time equal to one quarter (or 360 divided by 4) of the period. Hubner, U.S. Pat. No. 3,887,859, is a practical example of such an arrangement; see, in this connection, the windings 1a through 1d in the Figure of drawing.
In practice, it has been found that the construction of FIGS. 2 and 4 is subject to a difficulty which has limited its commercial applicability. The difficulty arises from the divided nature of the windings, and the unavoidable leakage inductance associated with them, separate and apart from the mutual inductance responsible for the development of the double supply voltage used to charge the capacitor C. When load current is present, energy is stored in the leakage inductance, at the time of commutation, it produces an additive voltage which tends to increase the charge on the capacitor and unless the capacitor has sufficient capability, this increase in charge can destroy the capacitor as well as apply excessive voltage stresses to semiconductor devices connected in the circuit. Greenwell, in U.S. Pat. No. 3,753,062, recognizes this problem and proposes a special winding arrangement as a solution.
Furthermore, the additive charge due to the leakage inductance increases proportional to load current without restriction. Increasing the size of the capacitor will contain the reactive energy, with a diminished voltage excursion, but this is not feasible for several reasons, one of which is economy. If the capacitor size is selected on the basis of the commutation requirements, that is to say, in practice to deliver maximum load current for about 40 microseconds, voltage overshoots of the order of 5 to 10 times supply voltage are encountered. For a nominal supply voltage of a few hundred volts, the needed semiconductor ratings and insulation stresses are increased into the kilovolt range.
The prior art has suggested suppression of these excessive voltages by use of surge clippers of various types (i.e., Greenwell's diodes 31); but in practice, the energy which they must be rated to continuously dissipate is substantial and wasteful of energy. Thus, it is one of the advantages of the present invention to avoid commutation voltage overshoot by regenerating leakage inductance stored energy to another of the load windings which are conducting. It is another object of the present invention to improve the load voltage waveform. Prior art structures such as shown in FIGS. 2 and 4 are inherently restricted to delivering a squarewave of load current over 180.degree. or 1/2 the period of the conduction sequence. See, for example, FIGS. 6A and 6B of Greenwell. The inverter of the present invention closely approximates the sinusoidal waveform by developing a three-phase stepped output with 120.degree. conduction in each phase (conduction in each load winding for 1/3 of the period of the conduction sequence). Achieving this object results in improved motor efficiency.